It is well known for lubricating oils to contain a number of surface active additives (including antiwear agents, dispersants, or detergents) used to protect internal combustion engines from wear, soot deposits and acid build up. Often, such surface active additives can have harmful effects on bearing corrosion or friction performance. As friction increases, fuel economy tends to decrease. A common antiwear additive for engine lubricating oils is zinc dialkyldithiophosphate (ZDDP). The addition of known friction modifiers (such as glycerol monooleate or oleyl tartrimide), is believed to balance or lower the coefficient of friction. However, friction modifiers may have deleterious effects as well as competing with the antiwear agent or bearing corrosion inhibitor (typically bearings containing lead and copper).
Various attempts have been made to reduce corrosion caused by ashless additives. These attempts include those disclosed in references described below. In general terms the references describe a variety of additives derived from a carboxylic acid, or a hydroxy-carboxylic acid.
U.S. Pat. No. 5,338,470 discloses alkylated citric acid derivatives obtained as a reaction product of citric acid and an alkyl alcohol or amine. The alkylated citric acid derivative is effective as a friction modifier.
U.S. Pat. No. 4,237,022 discloses tartrimides useful as additives in lubricants and fuels for effective reduction in squeal and friction as well as improvement in fuel economy.
U.S. Pat. No. 4,952,328 discloses lubricating oil compositions for internal combustion engines, comprising (A) oil of lubricating viscosity, (B) a carboxylic derivative produced by reacting a succinic acylating agent with certain amines, and (C) a basic alkali metal salt of sulphonic or carboxylic acid.
U.S. Pat. No. 4,326,972 discloses lubricant compositions for improving fuel economy of internal combustion engines. The composition includes a specific sulphurised composition (based on an ester of a carboxylic acid) and a basic alkali metal sulphonate.
U.S. Patent Application 60/862534 (PCT/US07/082057) discloses malonate esters.
International Publication WO 2005/087904 discloses lubricants containing hydroxy carboxylic acid and hydroxy polycarboxylic acid esters in combination with phosphorus-containing additives. The hydroxy polycarboxylic acid esters include tartaric acid and citric acid.
International Publication WO 2006/044411 discloses a low-sulphur, low-phosphorus, low-ash lubricant composition containing a tartrate ester, or amide having 1 to 150 carbon atoms per ester of amide group. The lubricant composition is suitable for lubricating an internal combustion engine.
EP 1 642 954 discloses a fluid composition comprising at least one hydroxy-substituted carboxylic acid. The hydroxy-substituted carboxylic acid provides at least one property chosen from rust inhibition, corrosion inhibition, improved lubricity, and improved lead compatibility. The hydroxy-substituted carboxylic acid is typically selected from hydroxycinnamic acid, 3-(4-hydroxyphenyl)propionic acid, 6-hydroxycaproic acid, 2-hydroxycinnamic acid, and 3-(2-hydroxyphenyl)propionic acid. Furthermore the composition disclosed therein is described as suitable for use in a transmission fluid. The transmission fluid is used an automatic transmission, continuous variable transmission, and/or a manual transmission.